The forming of can ends or shells for containers, namely aluminum or steel cans, is well-known in the art. Representative patents disclosing shell formation include Bulso U.S. Pat. Nos. 4,516,420 and 4,549,424. As is typically seen, a shell in its completed form includes a central panel, an outer rim than generally extends upward from the central panel called a chuckwall, and a thick, recessed groove that encircles the central panel between the central panel and the outer rim, commonly referred to as a countersink.
The countersink generally is used to enable attachment of a converted shell to a can base. Adversely, however, the countersink creates several disadvantages. The countersink provides a narrow groove in which excess liquid can be caught, which can be an annoyance to consumers. Further, the countersink can become an undesired receptacle for dirt or other unwanted substances, thereby potentially contaminating the contents of the can during use. On the financial front, the countersink uses excess metal material, thereby increasing cost.
Consequently, a need exists in the art for a can end or shell that does not include a countersink groove between the central panel and the outer rim, that is still attachable to the can body. One reference that attempts to accomplish this is United States Publication No. US 2004/0217780 to Turner et al. Turner attempts to achieve a shell that does not have a countersink through the creation of a z-fold between the central panel and the chuckwall. However, this application discloses a method that in practice forms a shell that is susceptible to failure. Turner uses a double action press with a double action die, and forms and sets the shell on a down stroke of the press without supporting the z-fold on the upstroke of the press. It was found, however that setting of the z-fold on the down stroke with a double action die creates a shell susceptible to failure, and the resultant shell's z-fold tended to unfold or was overly strain hardened. It is believed that the z-fold unfolds because the z-fold is not supported on the upstroke of the press. Also, it is believed that the z-fold is overly strain hardened because the z-fold is fully formed on the downstroke and not supported on the upstroke of the press. The overly strain hardened material of the z-fold can cause the material to fracture, rupture or cause leaks in the shell. Thus, a need remains for a countersink free shell with improved properties and manufacturability over the prior art.